SAR processing systems present advantages compared to state of the art systems, with respect to quality of the image obtained, ability to generate the image in a recursive manner with increasing resolution, and efficiency of the processing structure.
Imaging radar theory is based upon the concept of a synthetic aperture, and is extensively described in Asherman, D. et al. "Developments in radar imaging" IEEE Transactions on Aerospace and Electronic systems, Vol. AES-20, N4, July 1984, p. 363. A scene can be teledetected by a set of electromagnetic pulses transmitted by an airborne or a satellite borne radar. By means of suitable processing of the signals received by the radar, due to the scattering effect of the scene it is possible to reconstruct an electromagnetic reflectivity map as a function of space coordinates in a suitable reference system which contains the scene and the motion of the radar. The Asherman et al. article also introduces a spotlight operating mode.
In another reference, a paper by Munson, D.C. et al. entitled "A Tomographic Formulation of Spotlight Mode Synthetic Aperture Radar oc." IEEE, Vol. 71 Aug. 1983, pp. 917/925, a method for the processing of a signal is presented. In FIG. 7 in this paper, the processing of the signal received by the radar is performed after analogue to digital conversion along the following three processing steps: (i) interpolation of the data received and set on a polar grid so as to form new data on a cartesian plane; (ii) bidimensional inverse transform of data by means of the Fast Fourier Transform (FFT); and (iii) Extraction of the modules and display.
However, a system of the kind taught by Munson, et al. suffers the following drawbacks. First, it is impossible to obtain a high resolution image of sufficiently high quality. High resolution implies data acquisition over a wide radar observation angle. Therefore, the polar grid, which hosts the data to be processed, is quite different than the Cartesian grid onto which the data is interpolated. The interpolation error which arises introduces a deformation and defocusing on the final image. Second, the Munson et al. system possesses a high processing load due to the polar-Cartesian interpolation and to the bidimensional transformation. The number of multiplications required by the overall algorithm is proportional to N.sup.4, with N equal to the number of data sets on the polar grid. Lastly, the image scene is presented only upon completion of the acquisition of all data by the radar.
It is thus an object of the present invention to provide a radar antenna that radiates a beam which is continuously pointed onto a limited portion of the scene, despite the movement of the radar itself.
It is a further object of the present invention to provide a system that provides high resolution due to the illumination time of the radar towards a limited portion of the scene.
It is still a further object of the instant invention to provide a new processing apparatus that eliminates the drawbacks possessed by the Munson, et al. system, as described above.